Imaging the complex atmosphere of cool evolved stars Observing stars like the Sun

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1 Imaging the complex atmosphere of cool evolved stars Observing stars like the Sun Keiichi Ohnaka Max Planck Institute for Radio Astronomy Bonn, Germany Graphics: L. Calçada (ESO)

2 Mass-loss in cool evolved stars: long-standing problem Driving force is not identified Mira AGB stars Large-amplitude pulsation + radiation pressure or scattering on dust (S. Höfner s talk on Thursday) Non-Mira AGB stars (semi-regular & irregular variables) Much smaller pulsation amplitude. Mass-loss rate comparable to (optically bright) Miras = M 8 /yr Red supergiants Small pulsation amplitude. Mass-loss rate = M 8 /yr Convection? MHD process? Something else? How to tackle this problem? Physical properties of the outer atmosphere Hints for the wind acceleration mechanism

3 Schematic view of the outer atmosphere Extended molecular outer atmosphere (MOLsphere) K, 2 5 stellar radii Chromosphere Co-existence of multi-components MOLsphere ( K) Chromosphere (~ K) Possibly chromosphere confined to magnetic regions or shocks (McMurry & Jordan 2000; Harper & Brown 2006) Photosphere Dust formation

4 Spatially resolving stars like the Sun Stars with largest angular diameters (measured in 2.3 mm continuum) R Dor (AGB) : 47 mas Betelgeuse (RSG) : 43 mas Antares (RSG) : 37 mas Diffraction limit 25 mas (8m 1 mm) 20 mas 2400 Å) How to image detailed structures over the surface of stars like the Sun? Aperture-synthesis imaging by optical / infrared long-baseline interferometry

5 Spatially resolving stars like the Sun Very Large Telescope Interferometer (VLTI) Angular resolution = 2.3 mas 1.6 mm) 4 Unit Telescopes (8.2m, Fixed) 4 Movable 1.8m telescopes Change the array configuration depending on object s size/shape & science cases

6 Imaging surface structures Betelgeuse Imaging of Betelgeuse (M1.5I) 1.64 mm IOTA array (Haubois et al. 2009) 2 spots detected over the surface 20 mas Comparison with 3-D models (A. Chiavassa s talk this morning, P. Cruzalèbes S1-01) T Per 1 mas RS Per 1 mas T Per (M2I) & RS Per (M4I) 1.6 mm CHARA array (Baron et al. 2014) Bright spot on T Per Dark regions on RS Per Contrast ~ 25%

7 2014 Interferometry Imaging Beauty Contest (Slide from J. Monnier) Imaging the outer atmosphere 1.61 mm 1.67 mm 1.73mm R Car 10 mas H 2 O+CO H 2 O (weak) H 2 O IR spectrum of R Phe, proxy to R Car (Lançon et al. 2000) Mira star: R Car, phase = 0.84, VLTI / PIONIER instrument 1 or 2 spots detected over the surface Extended emission more prominent at 1.61 and 1.73 mm CO & H 2 O bands Other Miras (Ragland et al. 2008; Pluzhnik et al. 2009; Le Bouquin et al. 2009) Comparison with dynamical models: Wittkowski et al. (2011) Talk on Thursday Hillen et al. (2012) Poster S1-06

8 Spatially resolving the dynamics of the atmosphere Dynamics of the inhomogeneous atmosphere Directly probe the wind acceleration mechanism High spatial and high spectral resolution needed Optical / IR interferometry with high spectral resolution

9 First velocity-resolved imaging of the surface of stars Betelgeuse: 1-D imaging in the CO lines VLTI / AMBER Spectral resolution up to Individual CO lines resolved Probing the outer atmosphere 1-D image in each CO line Spatial resolution = 9.8 mas Beam size = 1/4 stellar size Spectral resolution = 6000 Ohnaka et al. (2011) Movie available at

10 First velocity-resolved imaging of the surface of stars Betelgeuse: 1-D imaging in the CO lines VLTI / AMBER Spectral resolution up to Individual CO lines resolved Probing the outer atmosphere 1-D image in each CO line Spatial resolution = 9.8 mas Beam size = 1/4 stellar size Spectral resolution = 6000 Ohnaka et al. (2011) Movie available at

11 First velocity-resolved imaging of the surface of stars Betelgeuse: 1-D imaging in the CO lines VLTI / AMBER Spectral resolution up to Individual CO lines resolved Probing the outer atmosphere 1-D image in each CO line Spatial resolution = 9.8 mas Beam size = 1/4 stellar size Spectral resolution = 6000 Ohnaka et al. (2011) Movie available at

12 First velocity-resolved imaging of the surface of stars Betelgeuse: Spatially resolved CO line spectrum 9.8 mas Spatially unresolved (= usual) spectrum Extended atmosphere appears as spikes

13 1-D imaging of Betelgeuse: Spectrum of the CO lines at each spatial position Movie available at

14 1-D imaging of Betelgeuse: Spectrum of the CO lines at each spatial position Movie available at

15 1-D imaging of Betelgeuse: Spectrum of the CO lines at each spatial position Stellar astrophysics a few steps closer to solar physics Movie available at

16 First velocity-resolved imaging of the surface of stars Betelgeuse: 1-D imaging in the CO lines Extended component up to 1.3 stellar radii First imaging of the outer atmosphere in CO lines Betelgeuse Blue wing: more extended, asymmetric Red wing: No extended component, symmetric Ohnaka et al. (2011)

17 First velocity-resolved imaging of the surface of stars Betelgeuse: Modeling the inhomogeneous velocity field Betelgeuse Ohnaka et al. (2011) First velocity-resolved imaging of the surface of a star Weak upwelling at 0 5 km/s & Strong downdrafts at km/s Similar velocity field spatially resolved on Antares too (Ohnaka et al. 2013) No systematic outflow, random, turbulent motions within ~1.5 stellar radii

18 Origin of the inhomogeneous velocity field Convection unlikely Observationally estimated density ~ g/cm 3 at 1.3 R 3-D convection model (Chiavassa et al. 2011) < g/cm 3 at 1.2 R Driven by MHD processes? Magnetic field detected on Betelgeuse ~1 G (Aurière et al. 2010) But no self-consistent simulation yet for red supergiants Pulsation? But variability amplitude is small, DV = mag

19 Velocity-resolved aperture-synthesis imaging M8 giant R Dor R Dor: M8 giant Diameter: 47 mas Beam = 6 x 10 mas Preliminary result: Ohnaka et al. (2014, in prep)

20 Velocity-resolved aperture-synthesis imaging M8 giant R Dor R Dor: M8 giant Diameter: 47 mas Beam = 6 x 10 mas Ohnaka et al. (2014, in prep) Extended emission stronger in the blue wing than in the red wing Possible detection of outward motions

21 Conclusions & Prospects Imaging of cool evolved stars reveals the complex atmosphere and the inhomogeneous velocity field Probing the velocity field at different heights using different molecular and atomic lines VLTI 2 nd generation instrument MATISSE (3 13 mm) SiO 4 mm, CO mm (G. Perrin s talk on Friday) Spatially resolving the chromosphere and its dynamics in the visible lines (Ha, Ca II triplet)

22 Thank you for your attention!